Implantable heart assist system and method of applying same

ABSTRACT

An extracardiac pumping system and method for using it supplement the circulation of blood, including the cardiac output, in a patient without any component thereof being connected to the patient&#39;s heart. A battery powered pump may be implanted subcutaneously and attached to an inflow conduit fluidly coupled to a patient&#39;s femoral artery via a subcutaneous anastomosis connection. The pump draws blood from the inflow conduit and discharges it through an outflow conduit fluidly coupled to a second peripheral artery via another subcutaneous anastomosis connection. The battery may be charged extracorporeally and the pump may be operated in either a continuous or pulsatile fashion in synchronization with the patient&#39;s heart. The conduits can be housed in a multi-lumen catheter and a reservoir may be provided which fluidly communicates with the inflow conduits. An additional feature may keep blood traveling extracorporeally within the system at or near body temperature. The system is capable of adjusting outflow from the pump to optimize the mixing of red blood cells in the aorta and thereby enhance oxygen delivery to tissues by minimizing blood cell concentration in the center of the aorta.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Application Ser. No.09/289,231 filed on Apr. 9, 1999, which is continuation-in-part of U.S.Application Ser. No. 09/166,005, now U.S. Pat. No. 6,200,260 B1, filedon Oct. 2, 1998, which claims the benefit of U.S. ProvisionalApplication Ser. No. 60/061,634, filed Oct. 9, 1997, which isincorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates generally to a system for assisting theheart and, in particular, to an extracardiac pumping system and a methodfor both supplementing the circulation of blood through the patient andfor enhancing vascular blood mixing using a minimally invasiveprocedure.

BACKGROUND OF THE INVENTION

During the last decade, congestive heart failure (CHF) has burgeonedinto the most important public health problem in cardiovascularmedicine. As reported in Gilum, R. F., Epidemiology of Heart Failure inthe U S., 126 Am. Heart J. 1042 (1993), four hundred thousand (400,000)new cases of CHF are diagnosed in the United States annually. Thedisorder is said to affect nearly 5 million people in this country andclose to 20 million people worldwide. The number of hospitalizations forCHF has increased more than three fold in the last 15 years.Unfortunately, nearly 250,000 patients die of heart failure annually.According to the Framingham Heart Study, the 5-year mortality rate forpatients with congestive heart failure was 75 per cent in men and 62 percent in women (Ho, K. K. L., Anderson, K. M., Kannel, W. B., et al.,Survival After the Onset of Congestive Heart Failure in Framingham HeartStudy Subject, 88 Circulation 107 (1993)). This disorder represents themost common discharge diagnosis for patients over 65 years of age.Although the incidence of most cardiovascular disorders has decreasedover the past 10 to 20 years, the incidence and prevalence of congestiveheart failure has increased at a dramatic rate. This number willincrease as patients who would normally die of an acute myocardialinfarction (heart attack) survive, and as the population ages.

CHF manifests itself primarily by exertional dyspnea (difficult orlabored breathing) and fatigue. Three paradigms are used to describe thecauses and therapy of CHF. The first views this condition in terms ofaltered pump function and abnormal circulatory dynamics. Other modelsdescribe it largely in terms of altered myocardial cellular performanceor of altered gene expression in the cells of the atrophied heart. Inits broadest sense, CHF can be defined as the inability of the heart topump blood throughout the body at the rate needed to maintain adequateblood flow, and many of the normal functions of the body.

To address CHF, many types of cardiac assist devices have beendeveloped. A cardiac or circulatory assist device is one that aids thefailing heart by increasing its pumping function or by allowing it acertain amount of rest to recover its pumping function. Becausecongestive heart failure may be chronic or acute, different categoriesof heart assist devices exist. Short of a heart transplant, at least twotypes of chronic heart assist systems have been developed. One typeemploys a full or partial prosthetic connected between the heart and theaorta, one example of which is commonly referred to as a LVAD—LeftVentricular Assist Device. With reference to FIG. 1 herein, one exampleof a LVAD 2 is shown. The LVAD comprises a pump and associated valves 4that draws blood directly from the apex of the left ventricle 6 anddirects the blood to the aortic arch 8, bypassing the aortic valve. Inthis application, the left ventricle stops functioning and does notcontract or expand. The left ventricle becomes, in effect, an extensionof the left atrium, with the LVAD 2 taking over for the left ventricle.The ventricle, thus, becomes a low-pressure chamber. Because the intentis to take over for the left ventricle, the LVAD operates by pumpingblood at cardiac rates. With an LVAD, oxygenated blood circulation isestablished sufficient to satisfy the demand of the patient's organs.Under these circumstances, however, continuous flow may not be desiredbecause the patient's arterial system is deprived of pulsatile waveflow, which is beneficial to certain parts of the patient.

Another type of chronic heart assist system is shown in U.S. Pat. No.5,267,940 to Moulder. Moulder describes a pump implanted into theproximal descending aorta to assist in the circulation of blood throughthe aorta. Because it is intended to pump blood flowing directly out ofthe heart, it is important that the Moulder device operate in a properlytimed, pulsatile fashion. If it is not operated in directsynchronization with the patient's heart, there is a risk that the pumpmight cause “carotid steal phenomenon” where blood is drawn away fromthe patient's brain through the carotid arteries when there isinsufficient blood in the left ventricle.

In addressing acute CHF, two types of heart assist devices have beenused. One is counterpulsatory in nature and is exemplified by anintra-aortic balloon pump (IABP). With an IABP, the balloon is collapsedduring isovolumic contraction, providing a reduced pressure againstwhich the heart must pump blood, thereby reducing the load on the heartduring systole. The balloon is then expanded, forcing bloodomnidirectionally through the arterial system. Another example of thisfirst type employs one or more collapsible chambers in which blood flowspassively into the chamber during systole, as is shown in U.S. Pat. No.4,240,409 to Robinson et al. The chamber is then collapsed and the bloodforcibly returned to the aorta. These devices simulate a chamber of theheart and depend upon an inflatable bladder to effectuate pumpingaction, requiring an external pneumatic driver. Moreover, they do notoperate as a continuous flow system, operating exclusively in pulsatilefashion.

A second type of acute assist device utilizes an extracorporeal pump,such as the Biomedicus centrifugal pump, to direct blood through thepatient while surgery is performed on the heart. In one example,described in U.S. Pat. No. 4,968,293 to Nelson, the heart assist systememploys a centrifugal pump in which the muscle of the patient isutilized to add pulsatility to the blood flow. The Nelson device is usedto bypass a portion of the descending aorta.

Another device, shown in U.S. Pat. No. 4,080,958 to Bregman et al.,utilizes an inflatable and collapsible bladder to assist in bloodperfusion during heart trauma and is intended to supplement aconventional heart-lung machine by imparting pulsatile actuation. In theprimary embodiment disclosed in Bregman, the balloon is controlled tomaintain sufficient pressure at the aortic root during diastole toensure sufficient blood perfusion to the coronary arteries. In analternative embodiment, a low resistance outlet from the aorta to theinferior vena cava is provided to reduce the aortic pressure duringsystole, thus, reducing the hemodynamic load on the left ventricle.

Other devices, such as that shown in U.S. Pat. No. 4,034,742 to Thoma,depend upon interaction and coordination with a mechanical pumpingchamber containing a movable pumping diaphragm. These devices areintended primarily for application proximate the heart and within thepatient's thorax, requiring major invasive surgery.

Many CHF devices are acutely used in the perioperative period. Forexample, U.S. Pat. No. 4,995,857 to Arnold discloses a perioperativedevice to pump blood at essentially cardiac rates during surgery whenthe heart has failed or has been stopped to perform cardiac surgery. TheArnold system temporarily replaces the patient's heart and lung andpumps blood at cardiac rates, typically 5 to 6 liters/min. Like allsystems that bypass the heart and the lungs, an oxygenator is required.Of course, with any system that includes an oxygenator, such as theconventional heart-lung machine, the patient cannot be ambulatory.

With early IABP devices, a polyurethane balloon was mounted on avascular catheter, inserted into the femoral artery, and positioned inthe descending aorta just distal to the left subclavian artery. Theballoon catheter was connected to a pump console that pumped helium orcarbon dioxide into the balloon during diastole to inflate it. Duringisovolumic contraction, i. e., during the brief time that the aorticvalve is closed and the left ventricle continues to contract, the gasused to actuate the balloon was rapidly withdrawn to deflate theballoon. This reduced the pressure at the aortic root when the aorticvalve opened. In contrast, during diastole, the balloon was inflated,causing the diastolic pressure to rise and pushing the blood in theaorta distally towards the lower part of the body (on one side of theballoon) and proximally toward the heart and into the coronary arteries(on the other).

The major advantage in such a counterpulsation device was systolicdeflation, which lowered the intra-aortic volume and pressure, reducingboth afterload and myocardial oxygen consumption. In other words, whenthe balloon is inflated, it creates an artificially higher pressure inthe aorta, which has the ancillary benefit of greater perfusion throughthe coronary arteries. When the balloon deflates, just before the aorticvalve opens, the pressure and volume of the aorta decrease, relievingsome of the hemodynamic burden on the heart. These physiologic responsesimproved the patient's cardiac output and coronary circulation,temporarily improving hemodynamics. In general, counterpulsation with anIABP can augment cardiac output by about 15%, this being frequentlysufficient to stabilize the patient's hemodynamic status, which mightotherwise rapidly deteriorate. When there is evidence of more efficientpumping ability by the heart, and the patient has moved to an improvedclass of hemodynamic status, counterpulsation can be discontinued, byslowly weaning while monitoring for deterioration.

Until 1979, all IABP catheters were inserted via surgical cutdown,generally of the femoral artery. Since then, the development of apercutaneous IABP catheter has allowed quicker, and perhaps safer,insertion and has resulted in more expeditious institution of therapyand expansion of clinical applications. Inflation and deflation of theballoon, however, requires a pneumatic pump that is sufficiently largethat it must be employed extracorporeally, thereby restricting thepatient's movements and ability to carry out normal, daily activities.LABP devices are, thus, limited to short term use, on the order of a fewdays to a few weeks.

As discussed above, a variety of ventricular assist pumping mechanismshave been designed. Typically associated with LVADs are valves that areused in the inlet and outlet conduits to insure unidirectional bloodflow. Given the close proximity of the heart, unidirectional flow wasnecessary to avoid inadvertent backflow into the heart. The use of suchvalves also minimized the thrombogenic potential of the LVAD device.

Typically, the pump associated with older LVADs was a bulky pulsatileflow pump, of the pusher plate or diaphragm style, such as thosemanufactured by Baxter Novacor or TCI, respectively. Given that the pumpwas implanted within the chest and/or abdominal cavity, major invasivesurgery was required. The pumps were typically driven through apercutaneous driveline by a portable external console that monitors andreprograms functions.

Alternatively, rotary pumps, such as centrifugal or axial pumps, havebeen used in heart assist systems. With centrifugal pumps, the bloodenters and exits the pump practically in the same plane. An axial pump,in contrast, directs the blood along the axis of rotation of the rotor.Inspired by the Archimedes screw, one design of an axial pump has beenminiaturized to about the size of a pencil eraser, although otherdesigns are larger. Despite its small size, an axial pump may besufficiently powerful to produce flows that approach those used witholder LVADs. Even with miniaturized pumps, however, the pump istypically introduced into the left ventricle through the aortic valve orthrough the apex of the heart, and its function must be controlled froma console outside the body through percutaneous lines.

All of these heart assist systems referred to above serve one or both oftwo objectives: (1) to improve the performance of a patient'soperative-but-diseased heart from the minimum, classified as NYHAC ClassIV, to practically normal, classified as I or 0; or (2) to supplementoxygenated blood circulation through the patient to satisfy organ demandwhen the patient's heart is suffering from CHF. With such systems,extreme pumping and large amounts of energy, volume, and heatdissipation are required.

Many of these heart assist systems have several general features incommon: 1) the devices are cardiac in nature; i. e., they are placeddirectly within or adjacent to the heart, or within one of the primaryvessels associated with the heart (aorta), and are often attached to theheart and/or aorta; 2) the devices attempt to reproduce pulsatile bloodflow naturally found in the mammalian circulatory system and, therefore,require valves to prevent backflow; 3) the devices are driven fromexternal consoles, often triggered by the electrocardiogram of thepatient; and 4) the size of the blood pump, including its associatedconnectors and accessories, is generally unmanageable within the anatomyand physiology of the recipient. Due to having one or more of thesefeatures, the prior art heart assist devices are limited in theireffectiveness and/or practicality.

Many of the above identified prior art systems, generally referred to asMechanical Circulatory Assist Devices, are not the only means, however,used to treat patients with congestive heart failure (CHF). Most CHFpatients are prescribed as many as five to seven different drugs toameliorate their signs and symptoms. These drugs may include diuretics,angiotensin converting enzyme (ACE) inhibitors, beta-blockers, cardiacglycosides, and peripheral vasodilators. The rationale forpharmacological intervention in heart failure include minimizing theload on the heart, improving the pumping action of the heart byenhancing the contractility of the muscle fibers, and suppression ofharmful neurohormonal compensatory mechanisms that are activated becauseof the decreased pumping function of the heart.

Noncompliance with what is often a complex drug regime may dramaticallyadversely affect the recovery of a CHF patient leading to the need forhospitalization and possibly morbidity and mortality. In addition, ACEinhibitors and diurectics can cause hypotension, which leads todecreased organ perfusion or an increasing demand on the heart to pumpmore blood. This leads to an inability, in many cases, to prescribe themost effective dosage of ACE inhibitors and a less than optimum outcomefor the patient. Patients suffering from CHF with the underlying causeof mitral valve insufficiency have been able to have their diureticsreduced following surgical repair of their mitral valve. This is due toan increased cardiac output and arterial pressures (as a result of thecorrection of the problem) resulting in more effective organ perfusion.With the reduction in the use of diuretics and the resultanthypotension, more effective dosages of ACE inhibitors can be used withmore favorable outcomes. In addition, it is easier for the patient tofollow a less complex drug regime, eliminating the costly and lifethreatening risks associated with noncompliance.

When blood flow through the coronary arteries falls below the levelneeded to provide the energy necessary to maintain myocardial function,due often to a blockage in the coronary arteries, a myocardialinfarction or heart attack occurs. This is a result of the blockage inthe coronary arteries preventing blood from delivering oxygen to tissuesdownstream of the blockage. The closer the blockage is to the coronaryostia, however, the more severe and life threatening the myocardialinfarction. The farther the location of the blockage is from thecoronary ostia, the smaller the area of tissue or myocardium that is atrisk. As the energy stored in the affected area decreases, myocardialcells begin to die. The larger the area that dies due to the loss ofoxygen, the more devastating the infarction. To reduce the area at risk,at least two known options are to either increase the oxygen supply tothe affected area or decrease the energy demands of the heart to prolongenergy stores until the blockage can be removed or reduced. Oneparticular method to increase blood flow, thereby increasing delivery ofoxygen to the affected area, is through a technique calledretroperfusion. This is accomplished by passing a cannula into eitherthe right or left ventricle (depending on the area of the blockage) andperfusing oxygenated blood retrograde up the coronary artery on thedownstream side of the blockage. Another method is to use drugs toincrease the force of contraction of the myocardium, creating increasedblood flow across the blocked area. Yet another method is to use drugs,such as pentoxifylline, aspirin, or TPA (tissue plaminogen activator),to reduce the viscosity of (thin out) the blood, inhibit plateletaggregation, or lyse thrombi (clots), respectively, thus, allowing moreblood to pass by the blockage. The goal of all of these methods is toincrease the delivery of oxygen to the tissue at risk.

The alternative option mentioned above is to reduce the energy demandsof the myocardium and increase the amount of time before irreversibledamage occurs. This can be accomplished by reducing the workload of theleft ventricle (which is the largest energy-consuming portion of theheart). An IABP is placed into the aorta and used as described above,resulting in a decreased afterload on the heart and increased perfusionof the coronary arteries and peripheral organs. An alternative way toreduce myocardial oxygen demand is to reduce the volume of blood theleft ventricle must pump. This can be accomplished by reducing the loadon the left ventricle, such as in a cardiopulmonary bypass or use of anLVAD. Unloading the left ventricle decreases the energy requirements ofthe myocardium and increases the amount of time before irreversibledamage occurs. This provides an opportunity to more effectively removeor decrease the blockage and salvage myocardial function. To besuccessful, each of these techniques must be implemented within a shortamount of time after the onset of a myocardial infarction. Thedisadvantage, however, is that each of these techniques can only beperformed in an emergency room or hospital setting. Unless the patientis already in the hospital when the myocardial infarction occurs, thereis usually some level of irreversible damage and subsequent loss ofmyocardial function.

It would be advantageous, therefore, to employ a heart assist systemthat avoids major invasive surgery and also avoids the use of peripheralequipment that severely restricts a patient's movement. It would also beadvantageous to have such a heart assist system that can be employed ina non-hospital setting for ease of treating acute heart problems underemergency conditions.

SUMMARY OF THE INVENTION

The object of the present invention is to address the aspect of CHF thatresults from altered pump function and abnormal circulatory dynamicswhile overcoming the limitations of prior art heart assist systems.Without functioning as a bypass to one or more of a patient's organs,the present invention comprises an extracardiac pumping system forsupplementing the circulation of blood through the patient without anycomponent thereof being connected to the patient's heart or primaryvessels. Thus, it is extracardiac in nature. With the ability to beapplied within a minimally invasive procedure, the present inventionsignificantly improves the condition of the patient suffering from CHF,resulting in the patient feeling much better, even where CHF continues.By supplementing the pumping action of the heart, in lieu of replacingit, the present system takes advantage of the pulsatile action of theheart, despite its weakened condition, to effectively deliver blood tobody organs that benefit from pulsatile delivery of oxygenated blood. Asa result, the present system is capable of being operated in acontinuous flow fashion or, if desired, in a pulsatile flow fashion.

An ancillary but important benefit of the present invention is theability to apply the present invention in such a way as to also reducethe pumping load on the heart, thereby potentially permitting the heartto recover during use. With the present invention, no bulky pump, valvesor oxygenator are required, and no thoracic invasion with major cardiacsurgery is required. Indeed, a significant advantage of the presentinvention is its simplicity while achieving extraordinary results inimproving the condition of a patient suffering from CHF.

The extracardiac system of the present invention preferably comprises,in one example, a rotary pump configured to pump blood through thepatient at subcardiac rates; i. e., at a flow rate significantly belowthat of the patient's heart. Other types of pumps or flow generatingmechanisms may be effective as well. Pumping the blood tends torevitalize the blood to a certain extent by imparting kinetic andpotential energy to the blood discharged from the pump. Importantly, thepreferred pump for the present invention pumping system is one thatrequires a relatively low amount of energy input, when compared to priorart pumps designed to pump at cardiac rates. The pump may be implantedor not, depending upon the capability, practicality, or need of thepatient to be ambulatory.

The present system also comprises an inflow conduit fluidly coupled tothe pump, to direct blood to the pump from a first peripheral bloodvessel, and an outflow conduit fluidly coupled to the pump, to directblood from the pump to a second peripheral blood vessel. The connectionof the inflow and outflow conduits to the respective blood vessels isperformed subcutaneously; not so deep as to involve major invasivesurgery. In other words, minimally subdermal. This permits applicationof the connections in a minimally-invasive procedure. Preferably, theconnections to the blood vessels are just below the skin or just belowthe first layer of muscle, depending upon the blood vessels at issue orthe location of the connection, although slightly deeper penetrationsmay be necessary for some patients or for some applications.

In an alternative embodiment, the present system is applied at a singlecannulated site using, for example, a multi-lumen catheter having atleast one lumen as an inflow lumen and a second lumen as an outletlumen. The multi-lumen catheter has an inflow port in fluidcommunicating with the inflow lumen. With this embodiment, blood isdrawn into the inflow port of the first lumen from a first peripheralblood vessel site, preferably the blood vessel into which themulti-lumen catheter is inserted. The output of the pump directs bloodthrough a second (outlet) port at the distal end of the second lumenthat is preferably located in a second peripheral vessel site. Thismethod accomplishes the same beneficial results achieved in thepreviously described embodiments, but requires only a single cannulatedsite, rather than two such sites. It should be appreciated that themulti-lumen catheter could be used in a manner where the outflow of thecannula is to the first peripheral site, while the inflow is drawn fromthe second peripheral vessel. Further still, it should be appreciatedthat the inflow could be positioned to draw blood from a peripheralvessel at the site of entry into the patient while the outflow could bepositioned in the aorta, proximate an arterial branch.

In one embodiment of the extracardiac system, the pump is a continuousflow pump, a pulsatile pump, and/or a pump that is configured togenerate flow in both a continuous and pulsatile format. The pump may beimplantable and is used to connect two peripheral arteries, such as thefemoral artery at the inflow and the left axillary artery at theoutflow, although other peripheral blood vessels are contemplated,including other arteries and/or veins, as well as any singular and/orcumulative combination thereof. An alternative embodiment employs both acontinuous flow and a pulsatile flow pump connected in parallel or inseries and operating simultaneously or in an alternating fashion. Yetanother alternative embodiment employs a rotary pump that iscontrollable in a synchronous copulsating or counterpulsating fashion,or in some out-of-phase intermediate thereof. In one application, it iscontemplated that the present invention be applied such that the heartexperiences a reduced pressure at the aortic root during systole(afterload) and/or a reduced left ventricular end diastolic pressure(pre-load), thus reducing the hemodynamic burden or workload on theheart and, thus, permitting the heart to recover.

It is contemplated that, where the entire system of the presentinvention is implanted, that it be implanted subcutaneously without theneed for major invasive surgery and, preferably, extrathoracically. Forexample, the pump may be implanted in the groin area, with the inflowconduit attached to the femoral or iliac artery proximate thereto andthe outflow conduit attached to the axillary artery proximate theshoulder. It is contemplated that the outflow conduit be applied bytunneling it under the skin from the pump to the axillary artery. Whereimplanted, the pump is preferably powered by an implantable powersource, such as for example a battery, that may be regeneratedexternally by an RF induction system or be replaced periodically, and/ora self-generating power source that, for example, draws energy from thehuman body (e. g., muscles, chemicals, heat).

The present invention also comprises a method for supplementing thecirculation of blood in the patient and potentially reducing theworkload on the heart of a patient without connecting any component tothe patient's heart. The inventive method comprises the steps ofimplanting a pump configured to generate blood flow at volumetric ratesthat are on average subcardiac, wherein the pump has an inflow andoutflow conduit attached thereto; connecting a distal end of the inflowconduit to a first peripheral blood vessel with a minimally-invasivesurgical procedure to permit the flow of blood to the pump from thefirst peripheral blood vessel of the patient; implanting the inflowconduit subcutaneously; connecting a distal end of the outflow conduitto a second peripheral blood vessel with a minimally-invasive surgicalprocedure to permit the flow of blood away from the pump to the secondperipheral blood vessel of the patient; and operating said pump toperfuse blood through the patient's circulatory system. Where the secondperipheral blood vessel is the axillary artery, the step of connectingthe distal end of the outflow conduit is performed in such a manner thata sufficient flow of blood is directed toward the hand to avoid limbischemia while ensuring that sufficient flow is directed toward theaorta without damaging the endothelial lining of the second peripheralblood vessel. The same concerns for avoiding limb ischemia and damage tothe endothelial lining would apply, however, regardless of the selectionof second peripheral blood vessel.

In one specific application, the pump is capable of synchronous controlwherein the step of operating the pump includes the steps of beginningdischarge of blood out of the pump during isovolumic contraction anddiscontinuing discharge of blood when the aortic valve closes followingsystole. Depending upon the patient and the specific arrangement of thepresent system, this specific method results in reduced afterload and/orpreload on the heart while also supplementing circulation. For example,in one embodiment, the first and second blood vessels are the femoraland axillary arteries, respectively.

In an alternative method of applying the present invention, the pump isnot implanted and the inflow and outflow conduits are connected to thefirst and second blood vessels percutaneously, using a readily-removableconnector, such as a cannula, to connect the distal ends of each conduitto the blood vessels.

An important advantage of the present invention is that it utilizes thebenefits of an IABP, without the requirement of extracorporeal equipmentor the need to have a balloon or similar implement partially obstructinga blood vessel. In addition to the benefits of an IABP, it also offersthe benefit of reducing the preload on the heart. The present inventionthus offers simplicity and long-term use.

Another important advantage of the present invention is its potential toenhance mixing of systemic arterial blood, particularly in the aorta,and thereby deliver blood with a higher oxygen-carrying capacity toorgans supplied by arterial side branches off of the aorta. Thisovercomes the problem of blood streaming in the descending aorta thatmay sometimes occur in patients suffering from low cardiac output orother ailments resulting in low blood flow. The lack of mixing of theblood within the descending aorta that may result from blood streamingcould lead to a higher concentration of red blood cells and nutrients inthe central region of the aorta and a decreasing concentration of redblood cells closer to the aortic wall. This could result in lowerhematocrit blood flowing into branch arteries from the aorta. Where itis desired to address the potential problem of blood streaming, a methodof utilizing the present invention may include taking steps to assesscertain parameters of the patient and then to determine the minimumoutput of the pump that ensures turbulent flow in the aorta, therebyenhancing blood mixing. One embodiment of that method includes the stepof determining the Reynolds number and the average Womersley number forthe flow through the descending aorta before and/or after applying thepresent inventive system to the patient and adjusting the pumpaccordingly.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the invention will now bedescribed with reference to the drawings, which are intended toillustrate and not to limit the invention.

FIG. 1 is a schematic view of a cardiac assist device, known as a leftventricular assist device, showing a bypass from the apex of the leftventricle to the aortic arch;

FIG. 2 is a schematic view of a first embodiment of the presentinvention, shown applied to a patient's circulatory system.

FIG. 3 is a schematic view of a second embodiment of the presentinvention, shown applied to a patient's circulatory system.

FIG. 4 is a schematic view of a variation of the first embodiment ofFIG. 2 shown implanted into a patient;

FIG. 5 is a schematic view of a third embodiment of the presentinvention, shown applied to a patient's circulatory system.

FIG. 6 is a schematic view of a fourth embodiment of the presentinvention, shown applied to a patient's circulatory system.

FIG. 7 is a schematic view of an inflow L-shaped connector, showninserted within a blood vessel.

FIG. 8 is a schematic view of a fifth embodiment of the presentinvention employing a multi-lumen catheter for single site applicationto a patient.

FIG. 9 is a schematic view of a sixth embodiment of the presentinvention showing a reservoir and a portable housing for carrying aportion of the invention directly on the patient.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings provided herein, a more detailed descriptionof the embodiments of the present invention is provided below. It shouldbe noted, however, that while some embodiments have all of theadvantages identified herein, other embodiments may only realize somebut not all of the advantages.

The present invention provides a heart assist system that isextracardiac in nature. In other words, the present inventionsupplements blood perfusion, without the need to interface directly withthe heart and aorta. Thus, no major invasive surgery is necessary to usethe present invention. The present invention also lessens thehemodynamic burden or workload on the heart by reducing the pressure atthe aortic root during systole (afterload) and/or reducing leftventricular end diastolic pressure and volume (preload).

With reference to FIG. 2, a first embodiment of the present invention 10is shown applied to a patient 12 having an ailing heart 14 and an aorta16, from which peripheral brachiocephalic blood vessels extend,including the right subclavian 18, the right carotid 20, the leftcarotid 22, and the left axillary 24. Extending from the descendingaorta is another set of peripheral blood vessels, the left and rightfemoral arteries 26, 28.

The first embodiment 10 comprises a pump 32, having an inlet 34 and anoutlet 36 for connection of flexible conduits thereto. The pump 32 ispreferably a rotary pump, either an axial type or a centrifugal type,although other types of pumps may be used, whethercommercially-available or customized. In either case, the pump should besufficiently small to be implanted subcutaneously and preferablyextrathoracically, for example in the groin area of the patient, withoutthe need for major invasive surgery. Because the present invention is anextracardiac system, no valves are necessary. Any inadvertent backflowthrough the pump and/or through the inflow conduit would not harm thepatient.

Regardless of the style or nature chosen, the pump 32 of the presentinvention is sized to generate blood flow at subcardiac volumetricrates, less than about 50% of the flow rate of an average healthy heart,although flow rates above that may be effective. Thus, the pump 32 ofthe present invention is sized and configured to discharge blood atvolumetric flow rates anywhere in the range of 0.1 to 3 liters perminute, depending upon the application desired and/or the degree of needfor heart assist. For example, for a patient experiencing advancedcongestive heart failure, it may be preferable to employ a pump that hasan average subcardiac rate of 2.5 to 3 liters per minute. In otherpatients, particularly those with minimal levels of heart failure, itmay be preferable to employ a pump that has an average subcardiac rateof 0.5 liters per minute or less. In yet other patients it may bepreferable to employ a pump that is a pressure wave generator that usespressure to augment the flow of blood generated by the heart.

In one embodiment, the pump selected is a continuous flow pump so thatblood perfusion through the circulation system is continuous. In analternative embodiment, the pump selected has the capability ofsynchronous actuation; i. e., it may be actuated in a pulsatile mode,either in copulsating or counterpulsating fashion.

For copulsating action, it is contemplated that the pump 32 would beactuated to discharge blood generally during systole, beginningactuation, for example, during isovolumic contraction before the aorticvalve opens or as the aortic valve opens. The pump would be static whilethe aortic valve is closed following systole, ceasing actuation, forexample, when the aortic valve closes.

For counterpulsating actuation, it is contemplated that the pump 32would be actuated generally during diastole, ceasing actuation, forexample, before or during isovolumic contraction. Such an applicationwould permit and/or enhance coronary blood perfusion. In thisapplication, it is contemplated that the pump would be static during thebalance of systole after the aortic valve is opened, to lessen theburden against which the heart must pump. The aortic valve being openencompasses the periods of opening and closing, wherein blood is flowingtherethrough.

It should be recognized that the designations copulsating andcounterpulsating are general identifiers and are not limited to specificpoints in the patient's heart cycle when the pump begins anddiscontinues actuation. Rather, they are intended to generally refer topump actuation in which the pump is actuating, at least in part, duringsystole and diastole, respectively. For example, it is contemplated thatthe pump might be activated to be out of phase from true copulsating orcounterpulsating actuation described herein, and still be synchronous,depending upon the specific needs of the patient or the desired outcome.One might shift actuation of the pump to begin prior to or afterisovolumic contraction or to begin before or after isovolumic expansion.

Furthermore, the pulsatile pump may be actuated to pulsateasynchronously with the patient's heart. Typically, where the patient'sheart is beating irregularly, there may be a desire to pulsate the pumpasynchronously so that the perfusion of blood by the extracardiacpumping system is more regular and, thus, more effective at oxygenatingthe organs. Where the patient's heart beats regularly, but weakly,synchronous pulsation of the extracardiac pump may be preferred.

The pump 32 is driven by a motor 40 and/or other type of drive means andis controlled preferably by a programmable controller 42 that is capableof actuating the pump in pulsatile fashion, where desired, and also ofcontrolling the speed or output of the pump. For synchronous control,the patient's heart would preferably be monitored with an EKG in whichfeedback would be provided the controller 42. The controller 42 ispreferably programmed by the use of external means. This may beaccomplished, for example, using RF telemetry circuits of the typecommonly used within implantable pacemakers and defibrillators. Thecontroller may also be autoregulating to permit automatic regulation ofthe speed, and/or regulation of the synchronous or asynchronouspulsation of the pump, based upon feedback from ambient sensorsmonitoring parameters, such as pressure or the patient's EKG. It is alsocontemplated that a reverse-direction pump be utilized, if desired, inwhich the controller is capable of reversing the direction of either thedrive means or the impellers of the pump. Such a pump might be usedwhere it is desirable to have the option of reversing the direction ofcirculation between two peripheral blood vessels.

Power to the motor 40 and controller 42 may be provided by a powersource 44, such as a battery, that is preferably rechargeable by anexternal induction source (not shown), such as an RF induction coil thatmay be electromagnetically coupled to the battery to induce a chargetherein. Alternative power sources are also possible, including a devicethat draws energy directly from the patient's body; e. g., the patient'smuscles, chemicals or heat. The pump can be temporarily stopped duringrecharging with no appreciable life threatening effect, because thesystem only supplements the heart, rather than substituting for theheart.

While the controller 42 and power source 44 are preferably pre-assembledto the pump 32 and implanted therewith, it is also contemplated that thepump 32 and motor 40 be implanted at one location and the controller 42and power source 44 be implanted in a separate location. In onealternative arrangement, the pump 32 may be driven externally through apercutaneous drive line. In another alternative, the pump, motor andcontroller may be implanted and powered by an extracorporeal powersource. In the latter case, the power source could be attached to theside of the patient to permit fully ambulatory movement.

The inlet 34 of the pump 32 is preferably connected to a flexible inflowconduit 50 and a flexible outflow conduit 52 to direct blood flow fromone peripheral blood vessel to another. The inflow and outflow conduits50, 52 may, for example, be formed from Dacron, Hemashield or Gortexmaterials, although other synthetic materials may be suitable. Theinflow and outflow conduits 50, 52 may also comprise biologic materialsor pseudobiological (hybrid) materials (e. g., biologic tissue supportedon a synthetic scaffold). The inflow and outflow conduits are preferablyconfigured to minimize kinks so blood flow is not meaningfullyinterrupted by normal movements of the patient or compressed easily fromexternal forces. In some cases, the inflow and/or outflow conduits maycome commercially already attached to the pump. Where it is desired toimplant the pump 32 and the conduits 50, 52, it is preferable that theinner diameter of the conduits be less than 25 mm, although diametersslightly larger may be effective.

In one preferred application of the present invention, the firstembodiment is applied in an arterial-arterial fashion; for example, as afemoral-axillary connection, as is shown in FIG. 2. It should beappreciated by one of ordinary skill in the art that an axillary-femoralconnection would also be effective using the embodiments describedherein. Indeed, it should be recognized by one of ordinary skill in theart that the present invention might be applied to any of the peripheralblood vessels in the patient.

The inflow conduit 50 has a first proximal end 56 that connects with theinlet 34 of the pump 32 and a second distal end 58 that connects with afirst peripheral blood vessel, which is preferably the left femoralartery 26 of the patient 12, although the right femoral artery or anyother peripheral artery may be acceptable. In one application, theconnection between the inflow conduit 50 and the first blood vessel isvia an end-to-side anastomosis, although a side-to-side anastomosisconnection might be used mid-stream of the conduit where the inflowconduit were connected at its second end to an additional blood vesselor at another location on the same blood vessel (neither shown).

Similarly, the outflow conduit 52 has a first proximal end 62 thatconnects to the outlet 36 of the pump 32 and a second distal end 64 thatconnects with a second peripheral blood vessel, preferably the leftaxillary artery 24 of the patient 12, although the right axillaryartery, or any other peripheral artery, would be acceptable. In oneapplication, the connection between the outflow conduit 52 and thesecond blood vessel is via an end-to-side anastomosis, although aside-to-side anastomosis connection might be used mid-stream of theconduit where the outflow conduit were connected at its second end toyet another blood vessel (not shown) or at another location on the sameblood vessel. Preferably, the outflow conduit is attached to the secondblood vessel at an angle that results in the predominant flow of bloodout of the pump proximally toward the aorta and heart, such as is shownin FIG. 2, while still maintaining sufficient flow distally toward thehand to prevent limb ischemia.

It is preferred that application of the present invention to theperipheral blood vessels be accomplished subcutaneously; i. e., at ashallow depth just below the skin or first muscle layer so as to avoidmajor invasive surgery. It is also preferred that the present inventionbe applied extrathoracically to avoid the need to invade the patient'schest cavity. Where desired, the entire extracardiac system of thepresent invention 10 may be implanted within the patient 12. In thatcase, the pump 32 may be implanted, for example, into the groin area,with the inflow conduit 50 connected subcutaneously to, for example, thefemoral artery 26 proximate the pump 32. The outflow conduit would betunneled subcutaneously through to, for example, the left axillaryartery 24. In an alternative arrangement, the pump 32 and associateddrive and controller could be temporarily fastened to the exterior skinof the patient, with the inflow and outflow conduits 50, 52 connectedpercutaneously. In either case, the patient may be ambulatory withoutrestriction of tethered lines.

It is contemplated that, where an anastomosis connection is not desired,a special connector may be used to connect the conduits 50, 52 to theperipheral blood vessels. With reference to FIG. 3, a second embodimentof the present invention is shown, wherein the inflow conduit 50 andoutflow conduit 52 are connected to the peripheral blood vessels viafirst and second connectors 68, 70 each comprising three-openingfittings. In the preferred embodiment, the connectors 68, 70 comprise anintra-vascular, generally-tee-shaped fitting 72 having a proximal end74, a distal end 76, and an angled divergence 78 permitting connectionto the inflow and outflow conduits 50, 52 and the blood vessels. Theproximal and distal ends 74, 76 of the fittings 72 permit connection tothe blood vessel into which the fitting is positioned. The angle ofdivergence 78 of the fittings 72 may be 90 degrees or less in eitherdirection from the axis of flow through the blood vessel, as optimallyselected to generate the needed flow distally toward the hand to preventlimb ischemia, and to insure sufficient flow and pressure toward theaorta to provide the circulatory assistance and workload reductionneeded while minimizing or avoiding endothelial damage to the vessel. Inanother embodiment, the connectors 68, 70 are sleeves (not shown) thatsurround and attach to the outside of the peripheral blood vessel where,within the interior of the sleeve, a port to the blood vessel isprovided to permit blood flow from the conduits 50, 52 when they areconnected to the connectors 68, 70, respectively.

Other types of connectors having other configurations are contemplatedthat may avoid the need for an anastomosis connection or that permitconnection of the conduits to the blood vessels. For example, it iscontemplated that an L-shaped connector be used if it is desired towithdraw blood more predominantly from one direction of a peripheralvessel or to direct blood more predominantly into a peripheral vessel.Referring to FIG. 7, an inflow conduit 50 is fluidly connected to aperipheral vessel, for example, the left femoral artery 26, using anL-shaped connector 310. The connector 310 has an inlet port 312 at aproximal end and an outlet port 314 through which blood flows into theinflow conduit 50. The connector 310 also has an arrangement of holes316 within a wall positioned at a distal end opposite the inlet port 312so that some of the flow drawn into the connector 310 is divertedthrough the holes 312, particularly downstream of the connector, as inthis application. A single hole in the wall could also be effective,depending upon size and placement. The connector may be a deformableL-shaped catheter percutaneously applied to the blood vessel or, in analternative embodiment, be connected directly to the walls of the bloodvessel for more long term application. By directing some blood flowdownstream of the connector during withdrawal of blood from the vessel,ischemic damage downstream from the connector may be avoided. Suchischemic damage might otherwise occur if the majority of the bloodflowing into the inflow connector were diverted from the blood vesselinto the inflow conduit. It is also contemplated that a connection tothe blood vessels might be made via a cannula, wherein the cannula isimplanted, along with the inflow and outflow conduits.

The advantage of discrete connectors is their potential application topatients with chronic CHF. A connector eliminates a need for ananastomosis connection between the conduits of the present inventionsystem and the peripheral blood vessels where it is desired to removeand/or replace the system more than one time. The connectors could beapplied to the first and second blood vessels semi-permanently, with anend cap applied to the divergence for later quick-connection of thepresent invention system to the patient. In this regard, a patient mightexperience the benefit of the present invention periodically, withouthaving to reconnect and redisconnect the conduits from the blood vesselsvia an anastomosis procedure each time. Each time it is desired toimplement the present invention, the end caps would be removed and theconduit attached to the connectors quickly.

In the preferred embodiment of the connector 70, the divergence 78 isoriented at an acute angle significantly less than 90° from the axis ofthe fitting 72, as shown in FIG. 3, so that a majority of the bloodflowing through the outflow conduit 52 into the blood vessel (e. g.,left axillary 24) flows in a direction proximally toward the heart 14,rather than in the distal direction. In an alternative embodiment, theproximal end 74 of the fitting 72 may have a diameter larger than thediameter of the distal end 76, without need of having an angleddivergence, to achieve the same result.

With or without a connector, with blood flow directed proximally towardthe aorta, the result may be concurrent flow down the descending aorta,which will result in the reduction of pressure at the aortic root. Thus,the present invention may be applied so to reduce the afterload on thepatient's heart, permitting at least partial if not complete CHFrecovery, while supplementing blood circulation. Concurrent flow dependsupon the phase of operation of the pulsatile pump and the choice ofsecond blood vessel to which the outflow conduit is connected.

While the present invention may be applied to create anarterial-arterial flow path, given the nature of the present invention,i. e., supplementation of circulation to meet organ demand, avenous-arterial flow path may also be used. For example, with referenceto FIG. 4, one embodiment of the present invention 10 may be applied tothe patient 12 such that the inflow conduit 50 is connected to aperipheral vein, such as the left femoral vein 80. In this arrangement,the outflow conduit 50 may be connected to one of the peripheralarteries, such as the left axillary 24. Arterial-venous arrangements arecontemplated as well. In those venous-arterial cases where the inflow isconnected to a vein and the outflow is connected to an artery, the pump32 should be sized to permit flow sufficiently small so thatoxygen-deficient blood does not rise to unacceptable levels in thearteries. It should be appreciated that the connections to theperipheral veins could be by one or more methods described above forconnecting to a peripheral artery. It should also be appreciated thatthe present invention could be applied as a venous-venous flow path,wherein the inflow and outflow are connected to separate peripheralveins. In addition, an alternative embodiment comprises two discretepumps and conduit arrangements, one being applied as a venous-venousflow path, and the other as an arterial-arterial flow path. When venousblood is mixed with arterial blood either at the inlet of the pump orthe outlet of the pump the ratio of venous blood to arterial bloodshould be controlled to maintain an arterial saturation of a minimum of80% at the pump inlet or outlet. Arterial saturation can be measuredand/or monitored by pulse oximetry, laser doppler, colorimetry or othermethods used to monitor blood oxygen saturation. The venous blood flowinto the system can then be controlled by regulating the amount of bloodallowed to pass through the conduit from the venous-side connection.

A partial external application of the present invention is contemplatedwhere a patient's heart failure is acute; i. e., is not expected to lastlong, or in the earlier stages of heart failure (where the patient is inNew York Heart Association Classification (NYHAC) functional classes IIor III). With reference to FIG. 5, a third embodiment of the presentinvention 110 is applied percutaneously to a patient 112 to connect twoperipheral blood vessels wherein a pump 132 and its associated drivingmeans and controls are employed extracorporeally. The pump 132 has aninflow conduit 150 and an outflow conduit 152 associated therewith forconnection to two peripheral blood vessels. The inflow conduit 150 has afirst end 156 and second end 158 wherein the second end is connected toa first peripheral blood vessel (e. g., femoral artery 126) by way of acannula 180. The cannula 180 has a first end 182 sealably connected tothe second end 158 of the inflow conduit 150. The cannula 180 also has asecond end 184 that is inserted through a surgical opening 186 or anintroducer sheath (not shown) and into the blood vessel source (e. g.,femoral artery 126).

Similarly, the outflow conduit 152 has a first end 162 and second end164 wherein the second end is connected to a second peripheral bloodvessel (e. g., left axillary artery 124) by way of a cannula 180. Likethe inflow cannula, the outflow cannula 180 has a first end 182 sealablyconnected to the second end 164 of the outflow conduit 152. The outflowcannula 180 also has a second end 184 that is inserted through surgicalopening 190 or an introducer sheath (not shown) and into the secondblood vessel (e. g., left axillary artery 124). By use of a percutaneousapplication, the present invention may be applied temporarily withoutthe need to implant any aspect thereof or to make anastomosisconnections to the blood vessels.

It is contemplated that a means for minimizing the loss of thermalenergy in the patient's blood be provided where the present inventivesystem is applied extracorporeally. Such means for minimizing the lossof thermal energy may comprise, for example, a heated bath through whichthe inflow and outflow conduits pass or, alternatively, thermal elementssecured to the exterior of the inflow and outflow conduits. Referring toFIG. 9, one embodiment comprises an insulating wrap 402 surrounding theoutflow conduit 152 having one or more thermal elements passingtherethrough. The elements may be powered, for example, by a battery(not shown). One advantage of thermal elements is that the patient maybe ambulatory, if desired. Other means that are known by persons ofordinary skill in the art for ensuring that the temperature of thepatient's blood remains at acceptable levels while travellingextracorporeally are also contemplated.

An alternative variation of the third embodiment may be used where it isdesired to treat a patient periodically, but for short periods of timeeach occasion and without the use of special connectors. With thisvariation, it is contemplated that the second ends of the inflow andoutflow conduits be more permanently connected to the associated bloodvessels via, for example, an anastomosis connection, wherein a portionof each conduit proximate to the blood vessel connection is implantedpercutaneously with a removable cap enclosing the eternally-exposedfirst end (or an intervening end thereof) of the conduit external to thepatient. When it is desired to provide a circulatory flow path tosupplement blood flow, the removable cap on each exposedpercutaneously-positioned conduit could be removed and the pump (or thepump with a length of inflow and/or outflow conduit attached thereto)inserted between the exposed percutaneous conduits. In this regard, apatient may experience the benefit of the present inventionperiodically, without having to reconnect and redisconnect the conduitsfrom the blood vessels each time.

Another embodiment of the present invention includes a plurality ofinflow and/or outflow conduits. For example, with reference to FIG. 6, afourth embodiment of the present invention 210 includes a pump 232 influid communication with a plurality of inflow conduits 250A, 250B and aplurality of outflow conduits 252A, 252B. Each pair of conduitsconverges at a generally Y-shaped convergence 296 that converges theflow at the inflow end and diverges the flow at the outflow end. Eachconduit may be connected to a separate peripheral blood vessel, althoughit is possible to have two connections to the same blood vessel atremote locations. In one arrangement, all four conduits are connected toperipheral arteries. Alternatively, one or more of the conduits could beconnected to veins. In the application shown in FIG. 6, inflow conduit250A is connected to left femoral artery 226 while inflow conduit 250Bis connected to left femoral vein 278. Outflow conduit 252A is connectedto left axillary artery 224 while outflow conduit 252B is connected toleft carotid artery 222.

It should be noted that the connections of any or all of the conduits tothe blood vessels may be via an anastomosis connection or via a specialconnector, as described above. In addition, the embodiment of FIG. 6 maybe applied to any combination of peripheral blood vessels that wouldbest suit the patient's condition. For example, it may be desired tohave one inflow conduit and two outflow conduits or vice versa. Itshould be noted that more than two conduits may be used on the inflow oroutflow side, where the number of inflow conduits is not necessarilyequal to the number of outflow conduits.

If desired, the present inventive system may further comprise areservoir that is either contained within or in fluid communication withthe inflow conduit. This reservoir is preferably made of materials thatare nonthrombogenic. Referring to FIG. 9, a reservoir 420 is positionedfluidly in line with the inflow conduit 150. The reservoir 420 serves tosustain adequate blood in the system when the pump demand exceedsmomentarily the volume of blood available in the peripheral blood vesselin which the inflow conduit resides until the pump output can beadjusted. The reservoir reduces the risk of excessive drainage of bloodfrom the peripheral blood vessel, which may occur when cardiac outputfalls farther than the already diminished baseline level of cardiacoutput, or when there is systemic vasodilation, as can occur, forexample, with septic shock. It is contemplated that the reservoir wouldbe primed with an acceptable solution, such as saline, when the presentsystem is first applied to the patient.

In an alternative embodiment, the present system comprises a multi-lumencatheter whereby the system may be applied by insertion at a singlecannulated site while the inflow and outflow conduits still fluidlycommunicate with peripheral vessels. Referring to FIG. 8, a multi-lumencatheter 510 could be inserted, for example, into the left femoralartery 26 and guided superiorly through the descending aorta to one ofnumerous locations. The blood could discharge, for example, directlyinto the descending aorta proximate an arterial branch, such as the leftsubclavian artery or, as shown in FIG. 2 by way of example, directlyinto the peripheral mesenteric artery 30. Preferably, the multi-lumencatheter 510 has an inflow port 512 that may be positioned within theleft femoral artery 26 when the catheter 510 is fully inserted so thatblood drawn from the left femoral artery is directed through the inflowport 512 into a first lumen 514 in the catheter. This blood is thenpumped through a second lumen 516 in the catheter and out through anoutflow port 520 at the distal end of the catheter 510. The outflow port520 may be situated within, for example, the mesenteric artery 30 suchthat blood flow results from the left femoral artery 26 to themesenteric artery 30. Preferably, where there is a desire for thepatient to be ambulatory, the multi-lumen catheter 510 should preferablybe made of material sufficiently flexible and resilient to permit thepatient to be comfortably move about while the catheter is indwelling inthe patient's blood vessels without causing any vascular trauma.

As explained above for several embodiments, one of the advantages of thepresent heart assist system is that it permits the patient to beambulatory. If desired, the system may be designed portably so that itmay be carried directly on the patient. Referring to FIG. 9, this may beaccomplished through the use of a portable case 610 with a belt strap612 to house the pump, power supply and/or the controller, along withcertain portions of the inflow and/or outflow conduits, if necessary. Itmay also be accomplished with a shoulder strap or other techniques, suchas a backpack or a fanny pack, that permit effective portability. Asshown in FIG. 9, blood is drawn through the inflow conduit 150 into apump contained within the portable case 610, where it is discharged intothe outflow conduit 152 back into the patient.

An important advantage of the present invention is its potential toenhance mixing of systemic arterial blood, particularly in the aorta.Such enhanced mixing ensures the delivery of blood with higheroxygen-carrying capacity to organs supplied by arterial side branchesoff of the aorta. A method of enhancing mixing utilizing the presentinvention preferably includes taking steps to assess certain parametersof the patient and then to determine the minimum output of the pumpthat, when combined with the heart output, ensures turbulent flow in theaorta, thereby enhancing blood mixing.

Blood flow in the aortic arch during normal cardiac output may becharacterized as turbulent in the end systolic phase. It is known thatturbulence in a flow of fluid through pipes and vessels enhances theuniform distribution of particles within the fluid. It is believed thatturbulence in the descending aorta enhances the homogeneity of bloodcell distribution in the aorta. It is also known that laminar flow ofviscous fluids leads to a higher concentration of particulates in thecentral portion of pipes and vessels through which the fluid flows. Itis believed that, in low flow states such as that experienced duringheart failure, there is reduced or inadequate mixing of blood cellsleading to a lower concentration of nutrients at the branches of theaorta to peripheral organs and tissues. As a result, the blood flowinginto branch arteries off of the aorta will likely have a lowerhematocrit, especially that flowing into the renal arteries, the celiactrunk, the spinal arteries, and the superior and inferior mesentericarteries. That is because these branches draw from the periphery of theaorta The net effect of this phenomenon is that the blood flowing intothese branch arteries has a lower oxygen-carrying capacity, becauseoxygen-carrying capacity is directly proportional to both hematocrit andthe fractional O₂ saturation of hemoglobin. Under those circumstances,it is very possible that these organs will experience ischemia-relatedpathology.

The phenomenon of blood streaming in the aorta, and the resultantinadequate mixing of blood resulting in central lumenal concentration ofblood cells, is believed to occur when the Reynolds number (N_(R)) forthe blood flow in the aorta is below 2300. To help ensure that adequatemixing of blood will occur in the aorta to prevent blood cells fromconcentrating in the center of the lumen, a method of applying thepresent invention to a patient may also include steps to adjust theoutput of the pump to attain turbulent flow within the descending aortaupstream of the organ branches; i. e., flow exhibiting a peak Reynoldsnumber of at least 2300 within a complete cycle of systole and diastole.Because flow through a patient is pulsatile in nature, and notcontinuous, consideration must be given to how frequently the blood flowthrough the aorta has reached a certain desired velocity and, thus, adesired Reynolds number. The method contemplated herein, therefore,should also include the step of calculating the average Womersley number(N_(W)), which is a function of the frequency of the patient's heartbeat. It is desired that a peak Reynolds number of at least 2300 isattained when the corresponding Womersley number for the same blood flowis approximately 6 or above.

More specifically, the method may comprise calculating the Reynoldsnumber for the blood flow in the descending aorta by determining theblood vessel diameter and both the velocity and viscosity of the fluidflowing through the aorta. The Reynolds number may be calculatedpursuant to the following equation: $N_{R} = \frac{V \cdot d}{\upsilon}$

where: V=the velocity of the fluid; d=the diameter of the vessel; andν=the viscosity of the fluid. The velocity of the blood flowing throughthe aorta is a function of the cross-sectional area of the aorta and thevolume of flow therethrough, the latter of which is contributed both bythe patient's own cardiac output and by the output of the pump of thepresent invention. Velocity may be calculated by the following equation:$V = \frac{Q}{\pi \quad r^{2}}$

where Q=the volume of blood flowing through the blood vessel per unittime, e. g., the aorta, and r=radius of the aorta. If the relationshipbetween the pump output and the velocity is already known orindependently determinable, the volume of blood flow Q may consist onlyof the patient's cardiac output, with the knowledge that that outputwill be supplemented by the subcardiac pump that is part of the presentinvention. If desired, however, the present system can be implementedand applied to the patient first, before calculating Q which wouldconsist of the combination of cardiac output and the pump output.

The Womersley number may be calculated as follows:$N_{W} = {r\sqrt{\frac{2{\pi\omega}}{\upsilon}}}$

where r is the radius of the vessel being assessed, ω is the frequencyof the patient's heartbeat, and ν=the viscosity of the fluid. For a peakReynolds number of at least 2300, a Womersley number of at least 6 ispreferred, although a value as low as 5 would be acceptable.

By determining (i) the viscosity of the patient's blood, which isnormally about 3.0 mm²/sec (kinematic viscosity), (ii) the cardiacoutput of the patient, which of course varies depending upon the levelof CHF, and (iii) the diameter of the patient's descending aorta, whichvaries from patient to patient but is about 21 mm for an average adult,one can determine the flow rate Q that would result in a velocitythrough the aorta necessary to attain a Reynolds number of at least 2300at its peak during the patient's heart cycle. Based upon thatdetermination of Q, one may adjust the output of the pump of the presentinvention to attain the desired turbulent flow characteristic throughthe aorta, enhancing mixing of the blood therethrough.

One may use ultrasound (e. g., echocardiography or abdominal ultrasound)to measure the diameter of the aorta, which is relatively uniform indiameter from its root to the abdominal portion of the descending aorta.Furthermore, one may measure cardiac output using a thermodilutioncatheter or other techniques known to those of skill in the art.Finally, one may measure viscosity of the patient's blood by using knownmethods; for example, using a capillary viscosimeter. It is expectedthat in many cases, the application of this embodiment of the presentmethod will provide a basis to more finely tune the system to moreoptimally operate the system to the patient's benefit. Other methodscontemplated by the present invention may include steps to assess otherpatient parameters that enable a person of ordinary skill in the art tooptimize the present system to ensure adequate mixing within thevascular system of the patient.

While the above description has explained the inventive features of theinvention as applied to various embodiments, it will be understood thatthe variations in the form and details of the apparatus or method may bemade by those of ordinary skill in the art without departing from thespirit of the invention. The scope of the invention is indicated by theappended claims herein, however, not by the foregoing description.

What is claimed is:
 1. An extracardiac pumping system for supplementingblood circulation in a patient without any component thereof beingconnected to the patient's heart, the extracardiac system comprising: apump configured to pump blood through the patient at subcardiac flowrates, said pump having an average flow rate that, during normaloperation thereof, is substantially below that of the patient's heartwhen healthy; an inflow conduit fluidly coupled to the pump to directblood to the pump from a first blood vessel branching off from a bloodvessel attached to the patient's heart, the inflow conduit having afirst end sized and configured to couple to the first blood vessel; anda reservoir fluidly coupled to the inflow conduit, said reservoir beingadapted to house a volume of blood from which the pump may draw blood.2. An extracardiac pumping system for supplementing blood circulation ina patient without any component thereof being connected to the patient'sheart, the extracardiac system comprising: a pump configured to pumpblood through the patient at subcardiac flow rates, said pump having anaverage flow rate that, during normal operation thereof, issubstantially below that of the patient's heart when healthy; an inflowconduit fluidly coupled to the pump to direct blood to the pump from afirst blood vessel; an outflow conduit fluidly coupled to the pump todirect blood from the pump to a second blood vessel; and a multi-lumencatheter housing at least two lumens, a first lumen fluidly connected tothe inflow conduit and a second lumen fluidly connected to the outflowconduit, at least one lumen also fluidly connected to a blood vesselbranching off from a blood vessel connected to the patient's heart;whereby the system is applied to the patient at a single site andwhereby blood may flow from the first blood vessel through the firstlumen in the multi-lumen catheter into the inflow conduit and whereblood may flow from the pump through the outflow conduit into the secondlumen and into the second blood vessel.
 3. A method of supplementingblood circulation in a patient using an extracardiac system comprising apump, an inflow conduit and an outflow conduit, said method of using theextracardiac system not involving connecting any component of theextracardiac system to the patient's heart, the method comprising thesteps of: inserting a multi-lumen catheter into the patient at a firstblood vessel branching off from a blood vessel that is connected to thepatient's heart, the multi-lumen catheter housing at least two lumens, afirst lumen in fluid communication with the first blood vessel and theinflow conduit, a second lumen fluidly connected to a second bloodvessel and the outflow conduit, said inflow and outflow conduits beingfluidly coupled to the pump; withdrawing blood from the first bloodvessel at subcardiac flow rates; and delivering blood from the pumpthrough the outflow conduit to the second blood vessel.
 4. The method ofclaim 3 wherein the first blood vessel from which blood is withdrawn isfrom the group consisting of subclavian, axillary, femoral and carotidarteries.
 5. An extracardiac pumping system for supplementing bloodcirculation in a patient without any component thereof being connectedto the patient's heart, the extracardiac system comprising: a pumpconfigured to pump blood through the patient at subcardiac flow rates,said pump having an average flow rate that, during normal operationthereof, is substantially below that of the patient's heart whenhealthy; an inflow conduit fluidly coupled to the pump to direct bloodto the pump from a peripheral blood vessel, the inflow conduit having afirst end comprising an intake port, the first end further comprising awall having at least one hole therein to permit the passage of bloodfrom the intake port through said hole; and an outflow conduit fluidlycoupled to the pump to direct blood from the pump to a second bloodvessel.
 6. An extracardiac pumping system for supplementing bloodcirculation in a patient without any component thereof being connectedto the patient's heart, the extracardiac system comprising: a pumpconfigured to pump blood through the patient at subcardiac flow rates,said pump having an average flow rate that, during normal operationthereof, is substantially below that of the patient's heart whenhealthy; an inflow conduit fluidly coupled to the pump to direct bloodto the pump from a first blood vessel, an outflow conduit fluidlycoupled to the pump to direct blood from the pump to a peripheral bloodvessel, the outflow conduit having a first end comprising an outletport, the first end further comprising a wall having at least one holetherein to permit the passage of blood through said hole.
 7. Anextracardiac pumping system for supplementing blood circulation in apatient without any component thereof being connected to the patient'sheart, the extracardiac system comprising: a pump configured to pumpblood through the patient at subcardiac flow rates, said pump having anaverage flow rate that, during normal operation thereof, issubstantially below that of the patient's heart when healthy; an inflowconduit fluidly coupled to the pump to direct blood to the pump from afirst peripheral blood vessel, the inflow conduit having a first endsized and configured to couple to the first peripheral blood vessel; anoutflow conduit fluidly coupled to the pump to direct blood from thepump to a second blood vessel; and means for minimizing heat loss fromthe blood that flows through the system.
 8. A method for improvingcross-sectional distribution of blood cells within a portion of thevascular system of a patient, the method comprising the steps of:supplementing the patient's cardiac output, said supplementing stepcomprising communicating blood at subcardiac rates between a peripheralblood vessel to a different blood vessel using an extracardiac systemthat does not have any component thereof being connected to thepatient's heart, said system including a pump; assessing flowcharacteristics in a portion of the patient's vascular system, andadjusting the output of the pump to foster turbulent blood flowdownstream of the heart.
 9. The method of claim 8 wherein the step ofassessing flow characteristics comprises determining the Reynolds number(N_(R)) for the flow through a portion of the patient's vascular system,said determining step comprising the steps of: measuring the diameter(d) of at least one blood vessel; determining the viscosity (ν) of theblood flowing through said vessel; and determining the velocity (V) ofthe blood flowing through said vessel; whereby N_(R) may be determinedby the following equation: $N_{R} = {\frac{V \cdot d}{\upsilon}.}$


10. The method of claim 9 wherein the step of determining the velocityof the blood flow is performed when said flow is being provided both bythe patient's heart output and the pump supplementing the patient'sheart.
 11. The method of claim 9, wherein the step of adjusting the pumpcomprises varying the discharge of the pump so that the peak Reynoldsnumber for the flow through the portion of the patient's vascular systembeing assessed is at least approximately
 2300. 12. The method of claim11, wherein the peak Reynolds number is 2300 when the correspondingWomersley number for the flow is above about
 5. 13. The method of claim8, wherein the portion of the vascular system being assessed is theaorta.
 14. A method for supplementing the circulation of blood in apatient while minimizing blood cell concentration at the center of ablood vessel in the patient's circulatory system, said method using anextracardiac heart assist system without connecting any component of thesystem to the patient's heart, the method comprising the steps of:connecting a distal end of an inflow conduit to a first blood vessel topermit the flow of blood to a pump from the first blood vessel;connecting a distal end of an outflow conduit to a second blood vesselto permit the flow of blood away from the pump to the second bloodvessel; operating the pump to perfuse blood through the patient'scirculatory system at subcardiac rates; determining the velocity ofblood flowing in the patient's aorta; and adjusting the pump outputaccordingly.
 15. The method of claim 14 wherein the step of operatingthe pump is performed prior to the step of determining the velocity ofblood flow.
 16. The method of claim 14 wherein the step of connecting adistal end of either the inflow or outflow conduit to a blood vesselcomprises connecting to a blood vessel that branches off from a bloodvessel directly connected to the patient's heart.
 17. An extracardiacpumping system for supplementing blood circulation in a patient withoutany component thereof being connected to the patient's heart, theextracardiac system comprising: a pump configured to pump blood throughthe patient at subcardiac flow rates, said pump having an average flowrate that, during normal operation thereof, is substantially below thatof the patient's heart when healthy; an inflow conduit fluidly coupledto the pump to direct blood to the pump from a first blood vessel thatbranches off from a blood vessel directly connected to the heart, theinflow conduit having a first end sized and configured to couple to thefirst blood vessel; and an outflow conduit fluidly coupled to the pumpto direct blood from the pump to a second blood vessel that branches offfrom a blood vessel directly connected to the heart, the outflow conduithaving a first end sized and configured to couple to the second bloodvessel.
 18. The extracardiac pumping system of claim 17 furthercomprising means for portably carrying a portion of the extracardiacsystem that resides extracorporeally on the patient, the carrying meansconfigured to carry at least the pump of said system.